Noise in general, and tonal noise in particular is very annoying. Low-frequency noise is very penetrating, travels very long distances and is difficult to attenuate using traditional passive control measures.
Passive noise control technology, which usually involves using absorptive materials or noise partitions, enclosures, barriers and silencers, can be bulky, ineffective and rather expensive at low frequencies. Active Noise Control (ANC), on the other hand, can be very efficient and relatively cheaper in reducing low-frequency noise.
Active Noise Control (ANC) is a technology using noise to reduce noise. It is based on the principle of superposition of sound waves. Generally, sound is a wave is travelling in space. If another, second sound wave having the same amplitude but opposite phase to the first sound wave can be created, the first wave can be totally cancelled. The second sound wave is named “antinoise”. Although the idea of ANC is not new, its practical application had to wait for the recent development of sufficiently fast electronic control technology.
A basic feed-forward active noise control system generally consists of a reference sensor (microphone), an electronic controller, a loudspeaker and an error sensor (microphone). The reference microphone picks up the information of the primary noise field and sends it to the electronic controller; the controller then drives the control loudspeaker to radiate the antinoise; the error microphone examines the control performance and modulates the controller for the best result.
An example of an active noise control system and method may be found in commonly-owned Patent Publication No. WO 2005/027338 (“388”). As shown and described therein, an active noise control (ANC) system may include an acoustic sensor (typically a microphone) to sense the noise energy and/or wave amplitude of a noise pattern produced by a noise source. The ANC system may also include an acoustic transducer (for example a speaker), and a controller to control the speaker to produce a noise destructive pattern to reduce or cancel the noise energy and/or wave amplitude of the noise pattern, for example within a reduced-noise zone. The controller may include an estimator to produce a predicted noise signal by applying an estimation function to one or more samples of noise signal. A noise error signal may be sensed by a second acoustic sensor (error-sampling microphones positioned in the reduced-noise zone.
Digital adaptive reduction of noise in the time domain is typically performed by sampling the analog output of a microphone that is appropriately positioned to sense the input noise. The sampled analog noise is then converted to digital format via an A/D converter, passed through an adaptive digital filter and then converted back to analog via a D/A converter before being output to a speaker. The analog output of a microphone is utilized as the input to the internal adaptive algorithm within the prior art noise reduction system.
A method of noise cancellation used in prior art systems places the microphone as close to the noise source as possible and the loudspeaker relatively far from the microphone so as to create a delay equal to the time for the noise to travel from the microphone to the speaker. This delay is intentionally created in order to match the internal signal processing time of the noise reduction system. The propagation time for the noise is configured to roughly match and compensate for the signal propagation time within the noise reduction system. This noise reduction method is particularly useful for cancellation of noise in a duct such as an air conditioning duct. The internal signal processing is performed during the time that it takes for the sound waves to travel from the microphone to the loudspeaker.
Another prior art noise reduction technique is to place the speaker close to the noise source rather than far away from it, place a second microphone in the desired quiet zone and adapt a digital filter utilizing the second microphone output. However, this method is useful for canceling repetitive noise only.
As electric/electronic devices get smaller and functional, the noise of cooling devices becomes important. Inside a desktop computer, there may be three (or more) fans. Usually there is a fan on the heat sink of the CPU, in the rear of the power supply unit, on the case ventilation hole, and may be on the graphics card, plus one on the motherboard chipset if it is a very recent one. The noise from a computer that annoys people is mostly due to cooling fans if the hard drive(s) is fairly quiet. When Intel Pentium processors were first introduced, there was no need to have a fan on the CPU at all, but most modern CPUs cannot function even for several seconds without a cooling fan, and some CPU's (such as Intel's Prescott core) have extreme cooling requirements, which often causes more and more noise. The type of fan used in a desktop computer is almost always an axial fan, while centrifugal fans are commonly used in laptop computers.
In many cases, for example, in blade chassis, RAID storage devices and the like (referred to herein as blade chassis) the noise level may exceed the level allowed according to the safety standards and regulations and in radical situations may even harm health. The noise emitted from standard fans normally used in blade chassis is characterized by one or several tones, such as at the low frequencies range (≦1000 Hz). Attempts were made to reduce the noise by passive treatment, for example, IBM 49P2694 Acoustic Attenuation Module. In order to reduce low frequencies range (≦1000 Hz) by means of passive treatment a substantial weight and size of material must be used. For example, to reduce a tone at 500 Hz by about 10 dBA, a muffler of more than 1 meter length and 30 centimeter diameter should be used. The passive means, which are currently being used, are not efficient for reduction of noise at low frequencies, particularly when dealing with fan noise involving airflow which cannot be blocked, without undesirable results (such as heat retention).